This is my planned use-case for the Soylent bars also.
My biggest concern is that it might not hold up in hot climates (see: Texas) during rides. I've already tried other bars that turn to mush in heat.
I also agree about the cost, since you can buy clif bars in bulk for less than $1.50/ea. And lately I've just been taking bananas with me on rides, which are like $0.10/ea.
I ordered a box earlier today, so we'll see how it works out. Worst case scenario, I keep them around for when I travel.